1. Field of the Invention
This invention relates to brake systems for a vehicle and more particularly to a brake force control device for a vehicle.
2. Description of the Related Art
A conventional brake force control device for a vehicle is disclosed in Japanese Patent Laid Open Publication No. 3(1991)-5270. A prior art brake pressure circuit of the prior art is shown in FIG. 4.
The brake force control device for a vehicle comprises a master cylinder 100 operated by a brake pedal 101 so as to generate a brake pressure, a wheel brake 102 including a brake cylinder which imparts a braking force to a wheel by the brake pressure, and a brake force control actuator 103 which includes a pressure control chamber connected to the brake cylinder, and a displacement actuator which varies the capacity of the pressure control chamber.
The master cylinder 100 is connected to the wheel brake 102 by a conduit 104 having a first normally open solenoid valve 105 and a second normally open solenoid valve 106.
A first one way valve 107 which prevents brake fluid from flowing to the wheel brake 102 from the master cylinder 100 is disposed in parallel arrangement with the first normally open solenoid valve 105. Further, a second one way valve 108 which prevents the brake fluid from flowing to the master cylinder 100 from the wheel brake 102 is disposed in parallel arrangement with the second normally open-solenoid valve 106.
The displacement actuator installed in the brake force control actuator 103 is driven by an electric motor 103a to vary the capacity of the pressure control chamber. The first solenoid valve 105 is closed to separate the wheel brake 102 and the pressure control chamber from the master cylinder 100 when the wheel brake 102 is under an antilock condition. On the other hand, the second solenoid valve 106 is closed when the wheel brake 102 is under an antislip condition. At these times, the displacement actuator increases and decreases the capacity of the pressure control chamber. Therefore, since the pressure control chamber is connected to the wheel brake 102, the brake force control actuator 103 increases and decreases the brake pressure in the wheel brake 102.
Under the antislip condition, the brake fluid quickly flows into the wheel brake 102 from the master cylinder 100 through the second one way valve 108 when the vehicle needs to be stopped. Further, under the antilock condition, the brake fluid rapidly flows into the master cylinder 100 from the wheel brake 102 through the first one way valve 107 when the brake pedal is released.
In accordance with this known system, the wheel brake 102 is connected to the master cylinder 100 through two solenoid valves 105, 106 and two one-way valves 107, 108. Therefore, the brake force control device must comprise eight solenoid valves and eight one-way valves when the brake pressure of each of the four wheel brakes is controlled independently. Consequently, the conventional brake force control device for a vehicle comprises a large number of components and the brake force control device is expensive.